Displacement with nodes doesn't work, do I need a Displacement modifier?
1) Material output Displacement input doesn't expect a color, it expects a value.  I don't have any idea what multiplying with what looks like .8 gray with a factor of 5 is going to do; that is highly unusual, to say the least.  Instead, use a simple math/multiply node to do this-- Blender will do an implicit RGB->BW conversion.

2) Rendered preview true displacement does not update with changes.  If you make changes to your displacement, you will need to leave and re-enter rendered preview.

3) I would recommend troubleshooting without any image texture plugged into your surface shader, with displacement set to "Both".  This will let you (and us) see if the issue is with the texture/ UV map, or due to some other issue.